Radio frequency power amplifiers (RFPAs) are used to translate RF signals, such as a modulated RF carrier in an RF transmitter, to higher RF powers. Often an RFPA will not have sufficient power gain to translate the low-power RF input signal to the desired or needed RF power. To address this limitation, RFPAs are often constructed from two or more RFPAs (or “stages”), as illustrated in FIG. 1. The total power gain G of the multi-stage RFPA 100 is determined by the product of the power gains of all of its N individual stages, where N denotes the number of stages. In other words, G=G1×G2× . . . GN, where G1, G2 and GN are the power gains of the individual stages PA-1, PA-2, and PA-N, respectively. When expressed in decibels (dB), the total power gain G(dB) is the sum of the individual power gains, in other words, G(dB)=G1(dB)+G2(dB)+ . . . +GN(dB).
The multi-stage RFPA 100 translates its input RF signal RFIN to higher RF power using direct current (DC) energy supplied to it from one or more external DC power supplies (indicated by the DC power supply voltages VDD1, VDD2, . . . , VDDN in FIG. 1). To best maintain linearity, its various stages PA-1, PA-2, . . . , PA-N are configured to operate as Class A, B or AB amplifiers. Unfortunately, these so-called “linear” RFPAs are very energy inefficient. Instead of converting all of the DC energy they receive into useful output RF power, they convert a large percentage of it into heat. Energy efficiency of any given stage is especially low during times the magnitude of its input RF signal is low, and particularly problematic in circumstances where the input RF signal RFIN is amplitude modulated. To prevent the peaks of any amplitude-modulated input RF signal from being clipped (i.e., to avoid loss of linearity and prevent distortion) as the input RF signal RFIN is amplified by the various stages PA-1, PA-2, . . . , PA-N, the output RF powers of one or more of the RFPA stages PA-1, PA-2, . . . , PA-N must be “backed off” Unfortunately, while backing off the output RF powers does help to maintain linearity, it further lowers the energy efficiency of the multi-stage RFPA 100.
Another problem with the multi-stage RFPA 100 is that it produces a significant amount of wideband noise (WBN). Mathematically, WBN can be expressed as follows: WBN(dB)=kT(dB)+G(dB)+NF(dB), where k is Boltzmann's constant (1.38×10−23 J/K), T is the absolute temperature of the RFPA's load in kelvins, G is the overall power gain of the multi-stage RFPA 100, and NF denotes what is known as the “noise figure.” The lower limit of WBN in any environment is determined by what is known as “thermal noise” (or “Johnson-Nyquist” noise), and is quantified by the “kT(dB)” term in the WBN formula. Thermal noise is generated by the agitation of charge carriers (electrons) present in electrical conductors, and is a type of “white” noise, meaning that its power spectral density is nearly constant across the frequency spectrum.
Thermal noise is usually modeled using a voltage source that represents the noise of a non-ideal resistor connected in series with an ideal noise-free resistor. According to this model, the “thermal noise power” generated by the non-ideal resistor is: P(dB)=10 log(kTB), where B is the bandwidth in hertz (Hz) over which the thermal noise is being considered. At room temperature (290K/17° C.), the thermal noise power in a B=1 Hz bandwidth is −174 dBm/Hz (where dBm means the decibel value is referenced to 1 milliwatt). Knowing this number, the “thermal noise floor” can be computed for any arbitrary channel bandwidth. For example, the thermal noise floor for the 200 kHz channel bandwidth used in the Global System for Mobile (GSM) communications standard is −174 dBm/Hz+10 log(200 kHz)=−121 dBm. This number represents the lowest noise power obtainable in a 200 kHz GSM channel.
The other two contributors to WBN, aside from kTB thermal noise, are the power gain G(dB) and noise figure NF(dB). Both operate to raise the noise floor above the kTB thermal noise floor. This effect on the WBN is illustrated in FIG. 2. The reason that the gain G(dB) affects the WBN is that every one of the various stages PA-1, PA-2, . . . , PA-N in the multi-stage RFPA 100 that has a positive, non-zero (in dB) gain not only translates its associated input RF signal to higher RF power, it/they also amplify(ies) any noise present at its/their inputs to higher power. Moreover, because the stages PA-1, PA-2, . . . , PA-N are not ideal devices and are themselves sources of noise, their mere presence increases WBN above the kTB thermal noise floor. These additional sources of noise are accounted for in the WBN formula above by the noise figure NF.
By definition, NF is a measure of how much a device (for example, an RFPA or a receiver) degrades the signal-to-noise ratio (SNR). In other words, NF=10 log(SNRinput/SNRoutput). NF is an intrinsic property of the physical device under consideration, and is a concept that is applicable to essentially any RF device or RF signal chain, including the multi-stage RFPA 100 depicted in FIG. 1. Because the SNR at the output of each stage PA-1, PA-2, . . . , PA-N is always smaller than the SNR at its input (SNRoutput<SNRinput), the NF of each stage, as well as the overall NF of the multi-stage RFPA 100, is always a positive number, i.e., can never be zero or negative. (Note that the NF term in the WBN formula above represents the overall NF of the multi-stage RFPA 100 and is determined by the individual NFs and gains G of all stages PA-1, PA-2, . . . , PA-N.)
Since at least one of the stages PAs PA-1, PA-2, . . . , PA-N of the multi-stage RFPA 100 must have a power gain greater than 0 dB (in order to translate the input RF signal to higher RF power), and because the multi-stage RFPA 100 always has a non-zero NF, as highlighted in the level diagram in FIG. 2, achieving low WBN in the output RF spectrum of the multi-stage RFPA 100 is often not possible. In some circumstances the WBN can be so high that it is impossible to satisfy the spectral mask requirements specified by an applicable communications standard. In other circumstances the WBN is so excessive that it completely swamps a proximate receive band (RX), as illustrated in FIG. 3.